This invention was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties.
1. Field of the Invention
The present invention relates to a probe for measuring magnetic flux and in particular, a probe for measuring time-varying magnetic fields produced in electric generators between the rotor poles and an iron stator.
2. Background of the Invention
A typical electric generator consists of a rotatable rotor and a stationary iron stator. An air gap is formed between the rotor and the stator. Electricity is generated as the rotor rotates relative to the iron stator. As the rotor rotates, substantial air currents, i.e., wind, are typically created.
A time-varying magnetic field (i.e., flux) is produced in the air-gap. The time-varying magnetic fields may be used to assess the health or status of the generator. For example, time-varying magnetic fields may indicate the existence of rotor shorted turns resulting from insulation failure between the individual windings of the electrical generator. Therefore, it can be advantageous to measure the time-varying magnetic fields produced in the air-gap.
One commercially available, conventional probe for measuring air-gap magnetic flux in electric generator employs a custom-wound wire coil. A second conventional available probe comprises a non-flexible, epoxy-encased probe. Both of these conventional probes tend to be relatively thick and massive and are mounted onto the surface of the stator. A voltage is generated in the probe by the magnetic field as the rotor rotates.
Conventional probes typically include a solid ground plane shield. The solid ground plane shield produces ground-loop-induced currents (i.e., eddy currents) when disposed in the air-gap of an electrical generator during operation. The ground-loop-induced currents can interfere with magnetic flux measurements.
One disadvantage with conventional commercial probes is that the probes are of size and mass that makes them susceptible to becoming unmounted by the considerable wind force generated in the air-gap by the rotating rotor. Consequently, these conventional probes can be dislodged and swept away inside the electric generator. Such dislodged or unmounted probes pose a potential damage threat to the generator.
A second disadvantage of conventional probes is that individual probes require calibration. Since conventional probes tend to be custom-wound coils of wire, calibration is necessary to ensure consistency in time-varying magnetic field detection. Thus, there is a cost associated with implementation of conventional probes to detect time-varying magnetic fields resulting from the necessary calibration steps.
As indicated above, an additional disadvantage with conventional probes is that the conventional ground-plane shield results in ground-loop-induced currents and, as stated, these induced currents affect the accuracy of measuring a magnetic flux present in the air-gap.
In accordance with the present invention, a flexible magnetic flux probe detects time-varying magnetic fields produced in the air-gap of an electric generator. The probe comprises a shield layer on a front layer and a printed circuit coil on the back layer. The flexible probe is adapted to be removably affixed to a stator of an electric generator. A time-varying magnetic field is detected by the probe as variations in voltage induced in the printed circuit coil during the electric motor operation (i.e., as the rotor spins).
According to one aspect of the invention, a flexible probe is provided for detecting time-varying magnetic fields produced in an electric generator. The electric generator has a rotor, an iron stator, and an air-gap formed therebetween. The flexible probe includes a flexible substrate having a front side and a back side. An electrostatic shield layer is formed on the front side of the substrate. A printed circuit having at least one coil (i.e., turn) is formed on the back side. A voltage is induced in the coil when the coil is exposed to a time-varying magnetic field. In one embodiment of the present invention, the electrostatic shield layer comprises traces of conductive material spaced parallel with gaps therebetween printed on the front side. The traces are connected together at one end.
According to another aspect of the invention, a device is provided for detecting time-varying magnetic fields in a system which includes an electric generator having a rotatable rotor, an iron stator, and an air-gap formed therebetween for producing the time-varying magnetic field when the rotor rotates. The device comprises a flexible printed circuit probe having a front side and a back side. The front side has an electrostatic shield formed thereon. The back side has a printed circuit coil layer formed thereon which includes at least one coil. The probe is removably fixable to the stator with the shield layer facing the rotor and the printed circuit layer facing the stator. A voltage is induced in the coil when the coil is exposed to a time-varying magnetic field.
According to yet another aspect of the invention, a method is provided for detecting time-varying magnetic fields in an electric generator having a rotatable rotor, an iron stator, and an air-gap formed therebetween which produces the magnetic fields when the rotor rotates. The method includes providing a flexible printed circuit probe having a front side and a back side in which the front side has an electrostatic shield layer formed thereon. The back side has a printed circuit coiled layer formed thereon and includes at least one coil. The flexible printed circuit probe is removably affixed onto the stator with the shield layer facing the rotor and the printed circuit layer facing the stator. The voltage generated in the printed circuit coil is then detected when the rotor rotates relative to the stator thereby responsive to inducing a voltage in the printed circuit coil layer.
An important feature of the present invention relates to the use of a flexible printed circuit board as a probe for detecting time-varying magnetic fields in the air-gap of an electric generator. This provides important advantages discussed in more detail hereinbelow.
Another feature of the present invention relates to the use of a simulated ground plane as a shield in a probe for detecting time-varying magnetic fields. The simulated ground shield is formed as printed traces of a conducting material spaced parallel with gaps therebetween and connected at one end. An important advantage of the simulated ground plane is that it reduces or prevents a ground-loop-induced current typically occurring in ground-plane shields of conventional probes used to detect air-gap magnetic fields.
Yet another important feature of the present invention relates to the use of low-cost multiple layer flexible printing circuit technology in producing a flexible probe that is inexpensive and disposable.
A further feature of the present invention is that the invention permits manufacturing technology that produces probes having uniform electrical and magnetic characteristics. As a result, these probes do not require calibration, thereby allowing the probes to be used in various locations within a machine with assurance of consistency between probes.
Still further advantages of the present invention result from the provision of a relatively thin, light and flexible probe. If the probe should become unmounted or otherwise separated from the stator, the thin, light and flexible probe is unlikely to cause damage to the electric generator. In this regard, the very low mass flexible substrate provides more flexibility in the probe mounting and contributes to the reduction or elimination of possible machine damage in the unlikely event that the probe should become unmounted.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.